1. Field of the Invention
The present invention relates to a method for controlling the wastegate in a turbocharged internal combustion engine.
2. Description of the Related Art
Some internal combustion engines are provided with a turbocharger supercharging system, which can increase the power developed by the engine by exploiting the enthalpy of exhaust gases for compressing the air aspirated by the engine, and thus increasing volumetric intake efficiency.
A turbocharger supercharging system typically includes a turbocharger provided with a turbine, which is arranged along an exhaust pipe to rotate at a high speed under the bias of the exhaust gases expelled by the engine, and with a supercharger. The supercharger is rotated by the turbine and is arranged along the air feeding pipe to compress the air aspirated by the engine. In a turbocharger supercharging system, the operating range of the turbocharger must be kept within a useful zone depending on the crank position for both functional reasons (i.e. to avoid irregular or low efficiency operation) and for structural reasons (i.e. to avoid damaging the turbocharger). In order to be able to limit the supercharging pressure (i.e. the pressure of the compressed air downstream of the supercharger), a bypass pipe regulated by a wastegate is typically arranged in parallel to the turbine. When the wastegate is opened part of the exhaust gas flows along the bypass pipe, and thus bypasses the turbine, and this decreases the rotation speed of the impeller, and thus decreases the supercharging.
A pneumatic actuator controlled by a regulating solenoid valve which regulates the intervention of the wastegate is used to control the wastegate. The pneumatic actuator includes a sealed shell, which internally supports a flexible membrane, which divides the sealed shell into two reciprocally, fluid-tight chambers. The flexible membrane is mechanically connected to a rigid rod, which controls the wastegate for controlling the opening and closing of the wastegate itself. A first chamber is connected to atmospheric pressure, while a second chamber is connected to the supercharging pressure and is connectable to atmospheric pressure by means of a pipe regulated by the regulating solenoid valve of the proportional type, which is adapted to divide the pipe between a closed position, in which the pipe is completely closed, and a maximum opening position.
A contrast spring, which is compressed between a wall of the shell and the flexible membrane, and which rests on the flexible membrane on the side opposite to the rod, is arranged in the first chamber. When the pressure difference between the two chambers is lower than an intervention threshold (determined by the preload of the contrast spring), the rod maintains the wastegate in a completely closed position. When the pressure difference between the two chambers is higher than the intervention threshold, the contrast spring starts to compress under the bias of the flexible membrane, which is thus deformed, determining a movement of the rod, which consequently moves the wastegate towards the opening position. By controlling the regulating solenoid valve, the second chamber can be connected to atmospheric pressure with a variable introduction gap, and thus the pressure difference between the two chambers can be regulated, which difference, in turn, determines the opening or closing of the wastegate. It is worth noting that until the difference between the supercharging pressure and the atmospheric pressure exceeds the intervention threshold (equal to the preload generated by the contrast spring divided by the flexible membrane area), the wastegate cannot be opened by the action exerted by the regulating solenoid valve (which can only reduce, and not increase, the pressure difference between the supercharging pressure and atmospheric pressure).
In the internal combustion engines of the type generally known in the related art, an objective supercharging pressure is generated, which is used to generate a control of the wastegate by adding an open loop contribution and a closed loop contribution: the open loop contribution is generated using an experimentally obtained control map, while the closed loop contribution is provided by a PID regulator. The PID regulator attempts to cancel a pressure error, which is typically represented by the difference between the objective supercharging pressure and the actual supercharging pressure measured by a sensor.
However, the preload generated by the contrast spring of the pneumatic actuator has a high construction dispersion, a considerable thermal drift and also a certain time drift. Furthermore, the pneumatic actuator has a considerable hysteresis. Thus, the behavior of the pneumatic actuator significantly varies between the opening movement and the opposite closing movement. Consequently, the map used for determining the closed loop contribution is strongly non-linear and the pursuing of the objective supercharging pressure is complicated. Thus, in internal combustion engines of the type generally known in the related art, the pursuing of the objective supercharging pressure tends to have high overshoots or undershooting (i.e. the actual supercharging pressure either exceeds or is even much lower than the objective supercharging pressure), and thus cause oscillations, particularly when the supercharging pressure surrounds the intervention threshold under which the wastegate cannot be opened by the action exerted by the regulating solenoid valve.
Overshoots (i.e. peaks) of the supercharging pressure are particularly annoying because they determine high strain (potentially dangerous in time) in the mechanical components of the internal combustion engine and because they may generate both noise perceivable by the vehicle occupants and corresponding undesired oscillations of the torque generated by the internal combustion engine.
In order to reduce overshoots, it is possible to reduce the integrative contribution of the PID regulator used to calculate the closed loop contribution of the wastegate control. However, this solution makes pursuing the objective pressure very slow, thus considerably increases the so-called turbo-lag, and often does not allow the system to reach the objective supercharging pressure (i.e. the actual supercharging pressure tends to the objective supercharging pressure but never reaches it).